Frequency stable oscillators are used in many microprocessors and digital applications. For example, oscillators are used in conjunction with a phase locked loop (PLL) to provide the clock frequency for central processing units (CPUs). Oscillators are also used in wireless application with PLLs to provide accurate carrier frequencies to meet the wireless standards for different protocols such as Bluetooth, GSM, and LTE. Typically, the oscillator frequency is controlled using a tank circuit (e.g., an inductor and capacitor in parallel or in series). One drawback of tank circuits is that standard inductors and capacitors have relatively low quality factors resulting in poor frequency stability over time and over temperature. Thus, a piezo crystal element is typically used. In its simplest form, a piezo crystal consists of a piezoelectric material between two plates and it oscillates at a precisely controlled frequency. Electrically, crystals are equivalent to tank circuits with very high quality factors. The crystals are typically sold as surface mount (SMT) components.
Recent miniaturization trends in wearables and the Internet of Things (IoT) require smaller and cheaper components. Piezo crystal components are relatively large and expensive (e.g., approximately $0.5-$1/piece). The piezo crystal components also typically have large Z-height (e.g., greater than or equal to 0.6 mm) which impacts the overall system cost and Z-height of a microelectronic device. The other oscillator solution that is currently available is based on Si-MEMS based resonators that can be made smaller in XY dimensions and have higher quality factors than passive networks; however their fabrication may be cost-prohibitive and they would still require assembly as discrete components to the system or board.